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Tabular display of interface MTUs

When I started exploring the details of MTU handling in Cisco IOS, I quickly got tired of analyzing various long printouts to extract the MTU sizes, so I wrote a Tcl script that display hardware, IP and MPLS MTUs in a tabular format. To install it on your router:
  1. Download it from my web site and copy it to your router's flash or NVRAM.
  2. Define an alias, for example alias exec mtu tclsh flash:displayMTU.tcl.

The script recognizes two parameters: the ip parameter displays only the interfaces that have IP configured and the mpls parameter displays only the MPLS-enabled interfaces.

Here is a sample printout from one of my lab routers:
Interface Hardware IP MPLS
FastEthernet0/0 1500 1600
FastEthernet0/1 1500
Serial1/0 1500 1500
Serial1/1 1380 1380 1430
Serial1/2 1500 1500
Serial1/3 1500
Loopback0 1514 1514
Tunnel0 1514 1476 1476

R2#mtu mpls
Interface Hardware IP MPLS
FastEthernet0/0 1500 1600
Serial1/1 1380 1380 1430
Tunnel0 1514 1476 1476
You can find more Tclsh-related information in the Tclsh on Cisco IOS tutorial. Sample Tclsh scripts are available in the Tclsh script library. If you need expert help in planning, developing or deploying Tclsh scripts in your network, contact the author.


  1. Very nice script...I installed it on a few of my routers and works flawlessly...once again it demostrates the power of scripting.
  2. Ivan:

    Can you recommend a good site for TCL training/resources?

    I am a dummy as far as TCL goes, but would like to learn about it.
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